Artificially coloured examples of the Longfin inshore squidCourtesy of Marine Biological Laboratory, Woods Hole, and Development

A squid-inspired infrared camouflage could one day help soldiers
escape detection on night-time missions.

The stealthy substance, created from a protein called reflectin,
can be dynamically retuned to different wavelengths of light,
allowing a hypothetical wearer to choose what types of light to
reflect or absorb.

"This tenability allows our films to reversibly disappear and
reappear when visualised with an infrared imaging camera," write
the University of California Irvine team behind the research, in a
paper published in Advanced Materials.

Squids and octopi, collectively described by the term
cephalopods, have been investigated for some time by researchers
interested in their incredible camouflaging abilities. Take for
example this video of an octopus turning white after being tossed
inside a boat of the same colour.

Octopus changes colour outside the wateroccy86

Mimicking their abilities could give militaries the edge when it
comes to stealth technology -- indeed in 2009 the US Office for
Naval Research awarded millions of dollars to Duke University for research
into squid and octopus camouflage.

One piece of the puzzle is the way the cells are structured on
their skin. The way they layer skin cells allows for something
called structural colour, which is when colour does not arise out
of a property of a cell itself, like the pigmentation in humans,
but rather colour arises from the way light refracts when it hits
the surface.

In the longfin inshore squid examined in the Advanced
Materials paper, layers of high refractive index (meaning
light travels slowly) cells are separated by layers of low
refractive index empty space -- by alternating the distance between
layers of cells, the squid is able to retune which wavelengths of
light it reflects and therefore change its colour.

To mimic this ability, the researchers took the protein found in
those high refractive index cells, reflectin, and grew it in a
bacteria culture. They then placed the resulting protein film on
top of thin films of graphene oxide and silicon dioxide.

By applying either water vapour or an acid solution, they were
able to swell the proteins on the film, which altered the
wavelengths of light the film reflected, allowing the researches to
"tune" the film.

The wavelength range of the film was found to be between 400nm
and 1200nm, well within the range of infrared imaging equipment,
which functions in the range 700nm to 1200nm.

This experimental research is of course a long way from being
used in the field, but in the paper the authors argue that it
represents "a crucial first step" towards developing new
bio-inspired stealth technology.